Squeeze the garlic cloves out of their skins into the bowl. Whisk until the mixture forms a paste. Reduce oven temperature to 350 degrees . Set aside giblets and neck for making Turkey Giblet Stock, if desired. Pat the turkey dry with paper towels. Loosen the skin over the breast and thigh meat. Rub the paste under the skin onto the breast meat and leg meat and a little inside the cavity. Tuck the wing tips under the turkey. Place the reserved squeezed lemon skins, thyme sprigs and 2 onion quarters in the cavity. (You may not use all the citrus skins.) Tie the legs together with kitchen string. Place the turkey breast-side up on a roasting rack set in a large roasting pan. Roast the turkey for 1 hour. Add 2 cups water and the remaining onion to the pan, tent with foil and continue roasting for 1 hour more. Baste the turkey with pan drippings and continue roasting, basting every 15 minutes or so, until an instant-read thermometer inserted into the thickest part of a thigh without touching bone registers 165°F, 1½ to 2 hours more. Add more water 1 cup at a time if the pan is dry. To prepare stock: Meanwhile, combine the reserved turkey neck and giblets (except liver), water, onion, carrot and celery in a large saucepan; bring to a boil. Add bay leaf, thyme and peppercorns. Reduce heat and simmer, skimming and discarding any foam, for 1 hour. Strain stock through a fine-mesh sieve into a medium bowl and let cool. Discard solids. If necessary, add enough water (or reduced-sodium chicken broth) to measure 4 cups stock. Transfer the turkey to a clean cutting board (reserve the pan juices and onions). Let the turkey rest while you make Citrus Gravy. To prepare gravy: Skim off any visible fat from the juices from the roasting pan. Whisk ½ cup of the stock and flour in a small bowl until smooth; set aside. Set the roasting pan over two burners on medium heat. Add wine; bring to a boil and cook, scraping up the browned bits, until the liquid is reduced by about half, 2 to 4 minutes. Add the remaining 3½ cups stock. Increase heat to medium-high; return to a boil, stirring often. Boil until the liquid is reduced by about half, 6 to 8 minutes. Whisk the reserved flour-stock mixture and add to the pan, whisking constantly, until the gravy thickens, 1 to 3 minutes. Stir in lemon juice (or lemon and orange juices). Remove from the heat and pour the gravy through a fine sieve into a large measuring cup. (Discard the solids.) Season with salt and pepper. Remove the string and carve the turkey. Serve with the gravy.